Free space optics communication apparatus and free space optics communication system

ABSTRACT

A free space optics communication apparatus which achieves high use efficiency of light and enables free control of the divergent angle of a light beam and the like is disclosed. The free space optics communication apparatus of the present invention has a light source which emits a light beam for performing wireless communication of information, and a minute mirror array unit which consists of a plurality of minute mirrors arranged in a matrix-like form and reflects the light beam emitted from the light source toward another apparatus. The apparatus also has a control unit which controls the directions of the individual minute mirrors constituting the minute mirror array unit independently. The control unit may be used to variably control the number of a plurality of mirror groups formed of a plurality of the minute mirrors in the minute mirror array unit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a free space opticscommunication apparatus which uses a light beam emerging into the spaceto perform wireless transmission of information.

[0003] 2. Detailed Description of the Related Art

[0004] In a conventional free space optics communication system, a freespace optics communication apparatus on the transmission side modulatesa transmission signal into a light signal which then emerges as a lightbeam toward a free space optics communication apparatus on the receptionside provided opposite thereto. The emerging light beam travels throughthe atmospheric space and is received at the free space opticscommunication apparatus on the reception side (another apparatus). Theother apparatus demodulates the light signal transmitted through thelight beam from the transmission side. In this manner, informationsignals are transmitted and received.

[0005] In the free space optics communication system as described above,the path of the light beam may vary due to fluctuations of theatmosphere or the like. In addition, the emerging direction of the lightbeam may vary since slight deformation readily occurs in a facilitywhere the free space optics communication apparatus is installed, forexample the rooftop of a building, due to a change in temperature.

[0006] When these external factors cause the light beam to arrive atvarying positions on the reception side, the level of reception isreduced at the free space communication apparatus on the reception side,and at worst, communication is interrupted.

[0007] In addition, in a case that deformation occurs in a facilitywhere the other apparatus on the reception side is installed, and theapparatus on the reception side is not disposed in a directionappropriate for the incident light beam, the level of reception isreduced and the reception may be impossible.

[0008] To solve the problems, a proposed free space optics communicationapparatus is configured to emerge a light beam such that the receptionside receives the light beam of large diameter to ensure a stable levelof reception on the reception side even when the beam path or theemerging direction of the beam varies.

[0009] Also, the apparatus on the reception side is designed to have acertain size of light-receiving angle (an angular range in which lightcan be received).

[0010]FIG. 12 shows an exemplary conventional free space opticscommunication system. In FIG. 12, a free space optics communicationapparatus 50 on the transmission side is shown on the left, while a freespace optics communication apparatus 71 on the reception side (anotherapparatus) is shown on the right. On the transmission side, a light beamemitted from a light source 72 is converted by an optical system 73 intoa light beam 76 which consists of substantially parallel rays withslight divergence. On the other hand, on the reception side, an opticalsystem 75 is designed to have a light-receiving angle slightly divergentas compared with parallel rays. The optical system 75 converges thelight beam which is received by a light-receiving element 74.

[0011] In general, to change the emerging angle or the receiving angleof a light beam in such a free space optics communication system, theoptical system, the light source, or the light-receiving element ismechanically moved to change their relative positional relationship. Inthis case, the relative positional relationship between thelight-emitting element 72 and the lens 73 is changed to vary thedivergent angle. Similarly, on the reception side, the relativepositional relationship between the light-receiving element 74 and thelens 75 is changed to vary the light-receiving angle.

[0012] In free space optics communication, bi-directional simultaneouscommunication is typically performed. A free space optics communicationapparatus has a transmitting unit and a receiving unit formed therein.When the apparatus has a transmitting optical axis and a receivingoptical axis coincident with each other, the apparatus can be providedwith an automatic tracking function of detecting the incident directionof light transmitted from another apparatus and matching the opticalaxis direction of the light beam from the other apparatus with thetransmitting and receiving optical axes of the apparatus itself. In thiscase, it is necessary to provide a means for changing the direction of atransmitting or receiving beam.

[0013] Possible methods of changing the direction of a transmitting orreceiving beam include changing the angle of the entire apparatus or theoptical system, providing an angular change for the beam by using amirror in the optical system, providing a positional change between thelens and the light-emitting element or the light-receiving element, orthe like.

[0014]FIG. 13 shows an example of the methods. A light beam emergingfrom a light-emitting element 81 a at a divergent angle 83 passesthrough a lens 82 and then emerges therefrom as a light beam 85consisting of substantially parallel rays.

[0015] A change in the beam emerging direction can be realized bychanging the relative position between the light-emitting element (lightsource) 81 a and the lens 82. When the light-emitting element 81 a isshifted to a position 81 b in FIG. 13, a light beam 84 incident on thelens 82 emerges into the space from the lens 82 as an emerging beam 86.The direction of the beam 86 is different from the direction of theaforementioned beam 85.

[0016] These methods are used not only to realize the automatic trackingfunction but also to align beam directions when the apparatus isinstalled.

[0017] Description is made for another structure which changes therelative positional relationship between the light-emitting element orthe light-receiving element and the optical system with reference toFIG. 14.

[0018] At the top in FIG. 14, a light beam is emitted from the endsurface of a light-emitting element 61 at a divergent angle 65. Thedivergent angle of the light beam is reduced by a lens 62 and then thelight beam is sent into the space at a divergent angle 68 which issubstantially collimated. For reducing the beam divergent angle toincrease the power density of light received at a receiving unit, thelight-emitting element 61 is moved on an optical axis in a directionaway from the lens 62 as shown at the bottom in FIG. 14. Of output lightfrom the light-emitting element 61, a light signal component incidenteffectively on the lens 62 in an angular range 66 results in a beamemerging angle 69 smaller than the angle 68. The light beam emergingfrom the lens 62 can become more collimated luminous flux.

[0019] However, since the divergent angle 65 of the light output fromthe light-emitting element 61 is constant, rays 67 which are noteffectively incident on the lens 62 are ineffective to cause a reductionin light power transmitted into the space through the lens 62.

[0020] If the relative positional relationship between thelight-emitting element 61 and the lens 62 is not changed and thecurvature of the lens 62 can be changed, the divergent angle can becontrolled effectively. However, it is difficult to form a lens or alens unit of which the curvature can be changed, and the structurethereof is complicated. In addition, a precise mechanism and accuratecontrol are required to avoid variations in the emerging direction oflight.

[0021] Typically, in the free space optics communication, the diameterof a received beam needs to be minimized at a reception point in orderto obtain sufficient light-receiving power. However, it is difficult toform a free space optics communication apparatus which can freely changethe beam emerging angle or the light-receiving angle for the reasons asdescribed above.

[0022] The shape of a beam may need to be controlled as well as the beamdivergent angle. In the free space optics communication apparatus,typically, the light-receiving angle is desirably set to be large on thereception side as described earlier. If a strong light source such asthe sun, a search light or the like is present behind the apparatus onthe transmission side, the strong light is incident on thelight-receiving element to prevent the intended light beam from beingreceived clearly.

[0023]FIG. 15 shows how the apparatus on the transmission side is viewedfrom the apparatus on the reception side. A strong light source 43 suchas the sun, a search light or the like, different from an inherent lightsource, partially enters a reception range 42 including an apparatus 41on the transmission side viewed from the apparatus on the receptionside.

[0024] Especially, the sunlight has much larger light power (energy)than the light power of a light beam from the apparatus 41 on thetransmission side. Thus, a communication failure may occur if even asmall amount of the sunlight enters the light-receiving range.

[0025] The conventional free space optics communication apparatus usinga lens takes measures against the incidence of the strong backgroundlight as mentioned above, for example by setting a smallerlight-receiving angle or changing the relative positional relationshipbetween the light-receiving element and the lens 62 according to thecircumstances as shown in FIG. 14. Such measures, however, cause aproblem that stable communication is unlikely to be ensured in thepresence of the fluctuations of the installation environment or thelike.

[0026] On the other hand, for controlling the shape of a beam pattern orthe like, it is necessary to provide an optical system responsible forthat control in addition to the optical system which controls the beamemerging angle or the light-receiving angle.

[0027] In addition, the shape (and the power density distribution) of abeam pattern subjected to control is uniquely determined by the providedoptical system and thus cannot be arbitrarily changed.

[0028] Furthermore, when a change in the beam emerging direction or thelight-receiving direction is intended, the structure shown in FIG. 13presents a problem that the light power output from the light-emittingelement 81 a is not effectively used, similarly to the method ofcontrolling the beam emerging angle or the light-receiving angle.

[0029] Specifically, a component 87 shown in FIG. 13 is not effectivelyincident on the lens 82 and is unnecessary since it is not transmittedinto the space. Also, if the component 81 is used as a light-receivingelement, the effective light-receiving angle 86 is smaller than theangle 85 which corresponds to the original light-receiving angle,thereby reducing the efficiency of reception.

[0030] Each of the optical systems formed for the respective purposes isa solid-state lens, a prism or the like. It is difficult to change theoptical systems separately or simultaneously at a high speed.

[0031] A conventional free space optics communication apparatus whichuses a light beam to perform point-to-multipoint informationtransmission radiates light toward a plurality of other apparatuses atremote locations to cover the entire range in which the otherapparatuses are present as in radio communication. In this case, thetransmission side needs to output light at a high level to allow theindividual other apparatuses to receive light with sufficient levels.

[0032] In the free space optics communication apparatus, however, thetransmission side has a limited output level of light from the viewpointof the lifetime of a light-emitting element or the like. It is thusimpossible to radiate light at a sufficient level in the entire range inwhich the other apparatuses are present.

[0033] In general, the free space optics communication apparatus whichuses a light beam to transmit information to a plurality of otherapparatuses at remote locations has a plurality of mirrors which reflectand send emerging light from the apparatus itself toward the pluralityof other apparatuses or reflect and take light sent from the pluralityof other apparatuses into a light-receiving section thereof. Since eachof the plurality of mirrors is set at an angle appropriate fortransmission and reception of light between the apparatus and each ofthe other apparatuses, the use of the mirrors enables efficienttransmission and reception of light in the free space opticscommunication apparatus for point-to-multipoint communication.

[0034] As described above, the free space optics communication apparatusneeds to have a margin for a certain deviation of the optical axis ofreceived light transmitted by any of the other apparatuses from theoptical axis of the light-receiving section thereof to avoid adeteriorated S/N ratio due to such a deviation caused by fluctuations ofthe air or the like.

[0035] The margin for a deviation is increased by setting a largerdivergent angle of emerging light on the transmission side. Typically,such a large divergent angle of an output light beam is achieved byproviding a concave surface for the plurality of mirrors which reflectand send emerging light to the plurality of other apparatuses or reflectand take light transmitted from the plurality of other apparatuses intothe light-receiving section.

[0036]FIG. 16 shows a conventional free space optics communicationapparatus. Reference numeral 901 shows a transmitting circuit, 902 alight-emitting element, 903 a polarization beam splitter which separatestransmission light and received light, and 904 a concave mirror unitwhich allocates light to other apparatuses. Reference numeral 905 showsa light-receiving element, and reference numeral 906 shows a receivingcircuit.

[0037] The transmitting circuit 901 converts a signal to be transmittedinto a signal which can be electro-optically converted. Thelight-emitting element 902 converts the signal into light which thenemerges as a light beam. The transmission light emerging from thelight-emitting element 902 passes through the polarization beam splitter903, is reflected by the concave mirror unit 904, and is directed towardeach of the plurality of other apparatuses.

[0038] On the other hand, a light beam transmitted from any of the otherapparatuses is reflected by the concave mirror unit 904, and sent to thepolarization beam splitter 903. The light beam reflected by thepolarization beam splitter 903 is converted into an electric signal bythe light-receiving element 905, and information included in the signalis received by the receiving circuit 906.

[0039] The concave mirror unit 904 has the structure as shown in FIG.17. Specifically, four concave mirrors 910 are arranged as in FIG. 17,in which each of the concave mirrors 910 is supported to be movable atany angle.

[0040] In the prior art, however, the number of the mirrors in the freespace optics communication apparatus is determined in the manufacturingstage of the apparatus. Thus, when the apparatus is actually installed,the number of mirrors is not always consistent with the number of otherapparatuses at that point.

[0041] If the number of the other apparatuses is smaller than the numberof the mirrors, a, certain number of mirrors are not used correspondingto the difference between them. In this case, since light from the lightsource is always made incident on the mirrors, light sent from theunused mirrors is unnecessary.

[0042] In addition, since the mirrors have the same curvatures, problemsarise both when the other apparatus is located at a shorter distance andwhen at a longer distance. For the other apparatus at a shorterdistance, received light has a small beam diameter and is incident onthe light-receiving element at too high a level, which may cause afailure of the light-receiving element. For the other apparatus at alonger distance, received light has an extremely large beam diameter anda smaller amount of light is received to reduce the margin for rain orthe like, in which case communication may be interrupted by a littlerain or fog. Thus, the communication range is limited in the prior art.

[0043] When the number of other apparatuses is larger than the number ofthe mirrors, an additional apparatus (apparatus on the transmissionside) is installed, or when the number of other apparatuses is smallerthan the number of the mirrors, light sent from extra mirrors isunnecessary. In addition, an available communication range is limited.

SUMMARY OF THE INVENTION

[0044] It is an object of the present invention to provide a free spaceoptics communication apparatus which achieves high use efficiency oflight and enables free control of the divergent angle of a light beamand the like.

[0045] To achieve the aforementioned objects, the free space opticscommunication apparatus according to the present invention has a lightsource which emits a light beam for performing wireless communication ofinformation, and a minute mirror array unit which consists of aplurality of minute mirrors arranged in a matrix-like form and reflectsthe light beam emitted from the light source toward another apparatus.The apparatus also has a control unit which controls the directions ofthe individual minute mirrors constituting the minute mirror array unitindependently. The control unit may be used to variably control thenumber of a plurality of mirror groups formed of a plurality of theminute mirrors in the minute mirror array unit.

[0046] These and other characteristics of the free space opticscommunication apparatus according to the present invention will beapparent from the following description of specific embodiments withreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a schematic view showing the structure of a free spaceoptics communication system which is Embodiment 1 of the presentinvention;

[0048]FIG. 2 is a diagram for explaining a minute mirror array unit ofthe free space optics communication system in Embodiment 1;

[0049]FIG. 3 is a diagram for explaining the light converging effect ofthe minute mirror array unit in Embodiment 1;

[0050]FIG. 4 is a diagram for explaining how to deal with unnecessarylight which is incident on the minute mirror array unit in Embodiment 1;

[0051]FIG. 5 is a schematic view showing the structure of a free spaceoptics communication apparatus which is Embodiment 2 of the presentinvention;

[0052]FIG. 6 is a diagram showing mirror groups arranged in a minutemirror array unit in the free space optics communication apparatus inEmbodiment 2 when four other apparatuses exist;

[0053]FIG. 7 is a diagram showing mirror groups arranged in the minutemirror array unit in the free space optics communication apparatus inEmbodiment 2 when two other apparatuses exist;

[0054]FIG. 8 is a diagram showing mirror groups arranged in the minutemirror array unit in the free space optics communication apparatus inEmbodiment 2 when two other apparatuses are located at a longer distanceand a shorter distance, respectively;

[0055]FIG. 9 is a section view of the mirror group in Embodiment 2;

[0056]FIG. 10 shows the structure of the minute mirror array unit inEmbodiment 2;

[0057]FIG. 11 shows the structure of another minute mirror array unit;

[0058]FIG. 12 is a diagram for explaining a conventional free spaceoptics communication system;

[0059]FIG. 13 is a diagram for explaining how the emerging direction ofa light beam is controlled in the conventional free space opticscommunication system;

[0060]FIG. 14 is a diagram for explaining how the diameter of a lightbeam is controlled in the conventional free space optics communicationsystem;

[0061]FIG. 15 is a diagram for explaining the case where the sun existsin the background on the transmission side in the free space opticscommunication system;

[0062]FIG. 16 is a diagram showing the structure of a conventional freespace optics communication apparatus; and

[0063]FIG. 17 is a schematic view of a concave mirror unit in theconventional free space optics communication apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Embodiments of the present invention are hereinafter describedwith reference to the drawings.

[0065] (Embodiment 1)

[0066]FIG. 1 shows the overview of a free space optics communicationsystem which is Embodiment 1 of the present invention. The system ofEmbodiment 1 uses a minute mirror array unit formed by arranging anumber of minute mirrors in a matrix-like form, the directions of whichcan be controlled independently, instead of the lens, prism or the likeused in the conventional system.

[0067] In FIG. 1, reference numeral 10 shows a free space opticscommunication apparatus on the transmission side (hereinafter referredto as “transmission-side apparatus”), while reference numeral 11 shows afree space optics communication apparatus on the reception side(hereinafter referred to as “reception-side apparatus”).

[0068] The transmission-side apparatus 10 comprises a light-emittingelement (light source) 12 such as a semiconductor laser or the like, aminute mirror array unit 14 which reflects a light beam (which isalready modulated in accordance with information to be transmitted)emitted from the light-emitting element 12 toward the reception-sideapparatus 11, and a control unit 18 which controls the directions ofindividual minute mirrors constituting the minute mirror array unit 14independently.

[0069] Reference numeral 21 shows a transmitting circuit which convertsa signal including information to be transmitted into a signal which canbe converted electro-optically by the light-emitting element 12. Thelight-emitting element 12 converts the signal into light which thenemerges therefrom.

[0070] On the other hand, the reception-side apparatus 11 comprises alight-receiving element 13, a minute mirror array unit 15 which reflectsthe light beam emerging from the transmission-side apparatus 10 towardthe light-receiving element 13, and a control unit 19 which controls thedirections of individual minute mirrors constituting the minute mirrorarray unit 15 independently.

[0071] Reference numeral 22 shows a receiving circuit. After thelight-receiving element 13 converts the light beam into an electricalsignal, the receiving circuit 22 receives information included in theelectrical signal.

[0072] Although not shown, the transmission-side apparatus 10 isprovided with a light-receiving element, a receiving circuit, and a beamsplitter which separates a light path for guiding a light beam reflectedby the minute mirror array unit 14 to the light-receiving element and alight path for guiding a light beam from the light-emitting element 12to the minute mirror array unit 14. The light-receiving element and thereceiving circuit are similar to those provided in the reception-sideapparatus 11.

[0073] On the other hand, although not shown, the reception-sideapparatus 11 is provided with a light-emitting element, a transmittingcircuit, and a beam splitter which separates a light path for guiding alight beam from the light-emitting element to the minute mirror arrayunit 15 and a light path for guiding a light beam reflected by theminute mirror array unit 15 toward the light-receiving element 13. Thelight-emitting element and the transmitting circuit are similar to thoseprovided in the transmission-side apparatus 10.

[0074] In other words, the transmission-side apparatus 10 and thereception-side apparatus 11 have the same structures, and each of themhas both a transmitting function and a receiving function of informationthrough a light beam.

[0075] In the following, however, description is made for the case whereinformation is transmitted from the transmission-side apparatus 10 tothe reception-side apparatus 11 through a light beam.

[0076] In the system configured above, a light beam emitted from thelight-emitting element 12 of the transmission-side apparatus 10 isreflected by the minute mirror array unit 14. The divergent angle (thatis, the diameter of the beam), the pattern (shape), the emergingdirection and the light-receiving direction of the reflected light beam16 can be set variably by the control unit 18 controlling the directionsof the respective minute mirrors constituting the minute mirror arrayunit 14. The directions of the respective minute mirrors are controlledsuch that the light beam 16 is sent to the space at a certain size ofdivergent angle.

[0077] On the other hand, a light beam 17 emerging from thetransmission-side apparatus 10 and incident on the reception-sideapparatus 11 is converged by the minute mirror array unit 15 of thereception-side apparatus 11 and incident on the light-receiving element13. At this point, the directions of the respective minute mirrors arecontrolled such that they have a certain light-receiving angle and asmall spot of light is formed on the light-receiving element 13.

[0078] The control units 18 of the transmission-side apparatus 10 andthe control units 19 of the reception-side apparatus 11 control thedirections of the respective minute mirrors to set the optimal divergentangle of the light beam, light-receiving angle, pattern, emergingdirection, and light-receiving direction under transmission conditionssuch as the distance (communication distance) between thetransmission-side apparatus 10 and the reception-side apparatus 11,scintillation, weather, the size and shape of a building present betweenthe transmission-side apparatus 10 and the reception-side apparatus 11,and vibrations of bases on which the transmission-side andreception-side apparatuses 10 and 11 are installed, respectively.Although not shown, a detector is provided for detecting the respectivecommunication conditions and sending a detection signal to the controlunits 18 and 19.

[0079] An operator may set the directions of the respective minutemirrors. Specifically, each of the control units 18 and 19 is providedwith an operation section, not shown, to allow control of the directionsof the respective minute mirrors with the control units 18 and 19 inresponse to operation of a switch, keyboard or the like provided for theoperation section.

[0080] For changing the beam emerging direction or the light-receivingdirection, a uniform angle change has only to be made in the individualminute mirrors of the minute mirror array units 14 and 15 in thestructure of FIG. 1. For example, when an angle change by 0.2 degrees isdesired in the emerging angle, control may be performed to move all theindividual minute mirrors in the same direction by 0.1 degrees.

[0081] The control of the emerging direction and light-receivingdirection can be realized by the minute mirror array units 14 and 15simultaneously with the control of the divergent angle of the light beamand the control of the beam pattern described earlier.

[0082] The system configured in this manner allows easy setting of thedivergent angle of the light beam, the light-receiving angle, thepattern, the emerging direction, and the light-receiving direction inaccordance with the installation conditions and environments of thetransmission-side apparatus 10 and the reception-side apparatus 11.

[0083] Next, the minute mirror array units 14 and 15 are described indetail with reference to FIG. 2. While FIG. 2 shows only the minutemirror array unit 14 provided for the transmission-side apparatus 10,the minute mirror array unit 15 provided for the reception-sideapparatus 11 has the same structure.

[0084] The minute mirror array unit 14 has a number of minute mirrors 14a arranged vertically and horizontally in a matrix-like form. The anglesof the individual minute mirrors can be set variably by an electricsignal from the control unit 18 driving an actuator, not shown. WhileFIG. 2 shows the minute mirror array unit 14 consisting of a total of225 minute mirrors 14 a of 15 by 15, the number of minute mirrors otherthan this may be employed, and the shape of the minute mirror array unitmay be circular or polygonal, rather than the square shown in FIG. 2.

[0085] The directions of the individual minute mirrors of the minutemirror array units 14 and 15 configured in this manner are controlledindependently, thereby causing each of the minute mirror array units 14and 15 to function as a converging optical system.

[0086]FIG. 3 shows the minute mirror array unit 14 shown in FIG. 2 takenalong a line A-A′. FIG. 3 shows the light-emitting element (point sourceof light) 12 disposed in front of the minute mirror array unit 14 fromwhich a light beam emerges as substantially parallel rays.

[0087] The directions of the individual minute mirrors 14 a of theminute mirror array unit 14 are controlled in this manner. Thus, a lightbeam which consists of substantially parallel rays or a light beam at alarge or small divergent angle can emerge therefrom.

[0088] On the other hand, the directions of the individual minutemirrors of the minute mirror array unit 15 in the reception-sideapparatus 11 are controlled such that the light-receiving element 13does not receive light from an unnecessary light source present as abackground of the transmission-side apparatus 10. When the unnecessarylight source moves as the sun does, the directions of the minute mirrorsmay be controlled in accordance with the movement of the unnecessarylight source.

[0089] Specifically, in a situation as shown in FIG. 15, the minutemirror array unit 15 of the reception-side apparatus 11 includes an area55 on which the light beam from the transmission-side apparatus 10 isincident and an area 56 on which the unnecessary light such as thesunlight is incident as shown in FIG. 4. In this case, the directions ofminute mirrors 57 in the area 56 on which the unnecessary light isincident (a dark color area in FIG. 4) are controlled not to face thelight-receiving element 13 to prevent the light reflected thereby frombeing converged to the light-receiving element 13. The minute mirrorsexcept for the minute mirrors 57 included in the unnecessary light area56 constitute a converging mirror for the light-receiving element 13.

[0090] In this manner, the individual minute mirrors are controlled toavoid incidence of light other than the intended incident light on thelight-receiving element 13 to prevent reception interference.

[0091] While Embodiment 1 has been described for the case where thetransmission-side apparatus and the reception-side apparatus communicatewith each other on a point-to-point basis, the system is applicable tothe case where a plurality of minute mirror array units are provided onthe transmission side to perform point-to-multipoint communication.

[0092] As described above, according to Embodiment 1, the minute mirrorarray units can be used to control the emerging angle of the light beam,the light-receiving angle, the emerging direction, and thelight-receiving direction. Thus, the light beam emitted from the lightsource can be sent to the transmission space with a minimized amount ofwasted component, and the divergent angle and the like of the light beamcan be controlled with a simple structure.

[0093] In addition, the pattern of the light beam can also be controlledsimultaneously. As compared with the conventional apparatus which usesan optical system such as a solid-state lens, the apparatus can besimplified and reduced in size.

[0094] (Embodiment 2)

[0095]FIG. 5 shows the overview of a free space optics communicationapparatus which is Embodiment 2 of the present invention. Although notshown, another free space optics communication apparatus with whichcommunication will be made has the structure similar to the free spaceoptics communication apparatus in FIG. 5. A free space opticscommunication system is formed of the free space optics communicationapparatus shown in FIG. 5 and a plurality of other apparatuses.

[0096] In FIG. 5, reference numeral 101 shows a transmitting circuit,102 a light-emitting element (light source), 103 a polarization beamsplitter which separates transmission light from received light, and 104a minute mirror array unit. Reference numeral 105 shows alight-receiving element, and reference numeral 106 shows a receivingcircuit. Reference numeral 108 shows a control unit which controls thedirections of individual minute mirrors constituting the minute mirrorarray unit 104 independently.

[0097] The transmitting circuit 101 converts a signal includinginformation to be transmitted into a signal which can beelectro-optically converted by the light-emitting element 102. Thelight-emitting element 102 converts the signal into light which thenemerges therefrom.

[0098] The light beam emerging from the light-emitting element 102passes through the polarization beam splitter 103, is reflected by theminute mirror array unit 104, and emerges toward the plurality of otherapparatuses (not shown).

[0099] On the other hand, a light beam transmitted from each of otherapparatuses is reflected by minute mirrors arranged in a matrix-likeform to constitute the minute mirror array unit 104. The reflected lightbeam is sent to the polarization beam splitter 103. The light beam isreflected by the polarization beam splitter 103, guided to thelight-receiving element 105, and converted into an electrical signal bythe light-receiving element 105. Then, information included in theelectrical signal is received by the receiving circuit 106.

[0100]FIG. 10 shows the minute mirror array unit 104 in detail. Theminute mirror array unit 104 is formed of a number of minute mirrors 104a arranged in a matrix-like form. The angle of each minute mirror 104 acan be set freely by an electrical signal from the control unit 108.

[0101] A mirror group which consists of a certain number of minutemirrors forming part of the minute mirror array unit 104 can function asa converging mirror (hereinafter, the mirror group is referred to as“pseudo converging mirror”) by setting the respective minute mirrors atappropriate angles as shown in FIG. 9.

[0102] Thus, an arbitrary number of the pseudo converging mirrors can beformed in the minute mirror array unit 104. In addition, the reflectivearea of a single pseudo converging mirror can be changed by changing thenumber of minute mirrors which constitute the pseudo converging mirror.

[0103] Additionally, the degree of light convergence of a pseudoconverging mirror can be changed by adjusting the angles of minutemirrors which constitute the pseudo converging mirror. Since the anglescorrespond to the curvature of a concave mirror, it is represented as“the curvature of a pseudo converging mirror” in the followingdescription.

[0104] The minute mirror array unit 104 configured as above is used inpoint-to-multipoint free space optics communication, and a plurality ofpseudo converging mirrors are formed in the minute mirror array unit104. Consequently, the following effects are expected.

[0105] For one thing, even when the number of other apparatuses ischanged, pseudo converging mirrors (in some cases, pseudo lightdiverging mirrors) can be formed corresponding to the number of theother apparatuses to allow efficient reception of light beams by theother respective apparatuses.

[0106] In addition, since a change in number of minute mirrorsconstituting a pseudo converging mirror can change the reflective areaof the pseudo converging mirror, an amount of transmission light can bedivided arbitrarily in accordance with the distances to the otherapparatuses. Specifically, the number of minute mirrors constituting apseudo converging mirror for another apparatus at a longer distance isset to be larger than the number of minute mirrors for another apparatusat a shorter distance.

[0107] Furthermore, the adjustment of the angles of minute mirrorsconstituting a pseudo converging mirror can change the curvature of thepseudo converging mirror, so that an appropriate beam diameter can beset for another apparatus.

[0108] Next, description is made for communication methods when fourother apparatuses exist and when two other apparatuses exist. When fourother apparatuses exist, four pseudo converging mirrors are formed inthe minute mirror array unit 104 as shown in FIG. 6. In FIG. 6, a pseudoconverging mirror formed at the upper right in the minute mirror arrayunit 104 is referred to as an A group, a pseudo converging mirror formedat the upper left as a B group, a pseudo converging mirror formed at thelower left as a C group, and a pseudo converging mirror formed at thelower right as a D group.

[0109] The pseudo converging mirrors are formed in groups in thismanner. The minute mirrors are controlled by electrical signals to setthe angles of the minute mirrors of the respective groups such that theA, B, C, and D groups reflect a light beam emitted from thelight-emitting element 102 toward the corresponding other apparatuses orreflect light beams emerging from other apparatuses toward thepolarization beam splitter 103 and the light-receiving element 105.

[0110] On the other hand, when two other apparatuses exist, two pseudoconverging mirrors are formed in the minute mirror array unit 104 asshown in FIG. 7. In FIG. 7, a pseudo converging mirror formed at the topin the minute mirror array unit 104 is referred to as an A group, and apseudo converging mirror formed at the bottom is referred to as a Bgroup.

[0111] The pseudo converging mirrors are formed in groups in thismanner. The minute mirrors are controlled by electrical signals to setthe angles of the minute mirrors of the respective groups such that theA and B groups reflect a light beam emitted from the light-emittingelement 102 toward the corresponding other apparatuses or reflect lightbeams emerging from other apparatuses toward the polarization beamsplitter 103 and the light-receiving element 105.

[0112] As seen from FIGS. 6 and 7, even when the number of the otherapparatuses is changed, all the emerging light from the apparatus can beallocated to the other apparatuses, which achieves high efficiency.

[0113] When communication is made with another apparatus at a longerdistance, an amount of light attenuation is increased in proportion todistance. For example, a comparison of amounts of attenuation incommunication with other apparatuses at distances of 1 km and 2 km whenit rains shows that the amount of attenuation at the distance of 2 km isequal to the square of the amount of attenuation at the distance of 1km. Thus, when communication is made with another apparatus at a longerdistance, a larger amount of light needs to emerge on the transmissionside than in communication with another apparatus at a shorter distancein order to ensure a margin for attenuation of light equal to that forthe other apparatus at the shorter distance. In contrast, whencommunication is made with another apparatus at a shorter distance, asmall amount of emerging light may be enough since an amount ofattenuation is small.

[0114] Description is made for communication methods performed whenother apparatuses are located at a longer distance and a shorterdistance, respectively. If two other apparatuses are installed at alonger distance and at a shorter distance, respectively, a mirror groupA corresponding to the other apparatus located at the longer distanceand a mirror group B corresponding to the other apparatus located at theshorter distance are arranged in the minute mirror array unit 104 asshown in FIG. 8. As can be seen, the number of minute mirrorsconstituting the mirror group A is larger than the number of minutemirrors constituting the mirror group B.

[0115] Most of the components of a light beam incident on the minutemirror array unit 104 are transmitted to the other apparatus at thelonger distance, while a minimal light component is transmitted to theother apparatus at the shorter distance. It is thus possible to providethe other apparatus at the longer distance with a margin for attenuationequal to that for the other apparatus at the shorter distance. Extralight more than necessary is not transmitted to the other apparatus atthe shorter distance. As a result, balanced allocation of light isachieved without waste.

[0116] Next, description is made for setting of the divergent angle of alight beam appropriate for other apparatuses at a longer distance and ashorter distance.

[0117] For example, consider the case where light beams at the samedivergent angles are transmitted to the other apparatus at the longerdistance and the other apparatus at the shorter distance. When light isoutput toward the other apparatus at the longer distance with asufficient margin for attenuation due to rain or the like, the diameterof the beam is small and the energy of the light is concentrated in theother apparatus at the shorter distance since the divergent angles arethe same. As a result, the light-receiving element receives light at ahigher level more than necessary.

[0118] In general, when the free space optics communication apparatus isused to perform communication with other apparatuses at a longerdistance and a shorter distance, the divergent angle of a light beam onthe transmission side is adjusted to provide the same beam diameters atthe other apparatuses regardless of the distances.

[0119] When communication is performed with the other apparatuses at alonger distance and a shorter distance in Embodiment 2, the curvaturesof the pseudo converging mirrors can be changed to adjust the divergentangles of the light beams such that the beams of the same diameter arereceived at the destinations regardless of the longer or shorterdistance. Thus, it is possible to transmit light of appropriate beamdiameter to other apparatuses at varying distances including longer andshorter distances.

[0120] While Embodiment 2 has been described for the case where each ofthe pseudo converging mirrors (mirror groups) is collectively formed inthe minute mirror array unit 104, minute mirrors constituting respectivemirror groups may be discretely arranged in a minute mirror array unit104′ as shown in FIG. 11, for example.

[0121] As described above, according to Embodiment 2, mirror groups canbe arbitrarily formed corresponding to the number of other apparatusesin performing point-to-multipoint free space optics communication. Thus,a single apparatus can support communication with various numbers ofother apparatuses, and an emerging light beam can be utilized withoutwaste.

[0122] In addition, since the numbers of minute mirrors constitutingrespective mirror groups can be changed or the curvatures of the mirrorgroups can be changed in accordance with the distances to the respectiveother apparatuses, information can be transmitted and received reliablyto and from the other apparatuses at varying distances without waste.

[0123] While Embodiments 1 and 2 have been described for the free spaceoptics communication apparatus which has both the transmitting functionand the receiving function of a light signal (light beam), the systemmay be formed by combining a free space optics communication apparatuswhich has only the transmitting function with a free space opticscommunication apparatus which has only the receiving function.

[0124] While preferred embodiments have been described, it is to beunderstood that modification and variation of the present invention maybe made without departing from the sprit or scope of the followingclaims.

What is claimed is:
 1. A free space optics communication apparatuscomprising: a light source which emits a light beam for performingwireless communication of information; a minute mirror array unit whichconsists of a plurality of minute mirrors arranged in a matrix-like formand reflects the light beam emitted from the light source toward anotherapparatus; and a control unit which controls directions of theindividual minute mirrors constituting the minute mirror array unitindependently, wherein at least one of a divergent angle of thereflected light beam, a pattern of the light beam, and a reflectingdirection of the light beam can be changed by the control unitcontrolling the directions of the individual minute mirrors.
 2. The freespace optics communication apparatus according to claim 1, furthercomprising: a light-receiving element which receives a light beam forperforming wireless communication of information, wherein the minutemirror array unit reflects the light beam emerging from the otherapparatus toward the light-receiving element.
 3. A free space opticscommunication system comprising: a plurality of the free space opticscommunication apparatuses according to claim 1; another apparatus whichhas at least a receiving function of receiving a light beam emergingfrom the free space optics communication apparatus.
 4. The free spaceoptics communication apparatus according to claim 1, wherein the controlunit variably controls the number of a plurality of mirror groups formedof the plurality of minute mirrors in the minute mirror array unit, andthe plurality of mirror groups reflect the light beam from the lightsource toward a plurality of other apparatuses.
 5. The free space opticscommunication apparatus according to claim 4, wherein the control unitsets a larger number of the minute mirrors constituting the mirror groupcorresponding to the other apparatus at a longer distance.
 6. A freespace optics communication apparatus comprising: a light source whichemits a light beam for performing wireless communication of information;a minute mirror array unit which consists of a plurality of minutemirrors arranged in a matrix-like form and reflects the light beamemitted from the light source toward another apparatus; and a controlunit which controls directions of the individual minute mirrorsconstituting the minute mirror array unit independently, wherein thecontrol unit changes the degree of light convergence of the light beamprovided by the plurality of minute mirrors in accordance with adistance to the other apparatus.
 7. The free space optics communicationapparatus according to claim 6, wherein the control unit variablycontrols the number of a plurality of mirror groups formed of theplurality of minute mirrors in the minute mirror array unit, and theplurality of mirror groups reflect the light beam from the light sourcetoward a plurality of other apparatuses.
 8. The free space opticscommunication apparatus according to claim 4 or 7, further comprising: alight-receiving element which receives a light beam for performingwireless communication of information, wherein each of the mirror groupsreflects the light beam emerging from a plurality of locations away fromthe free space optics communication apparatus toward the light-receivingelement.
 9. A free space optics communication system comprising: thefree space optics communication apparatus according to clam 4 or 7, anda plurality of other apparatuses, each of which has at least a receivingfunction of receiving the light beam emerging from the free space opticscommunication apparatus.